A so-called electroconductive glass in which an indium oxide thin film is formed on a glass as a transparent electroconductive thin film has been so far well known, but the electroconductive glass, the base of which is glass, is poor in flexibility and processability, and may be undesirable in some applications. Thus, in recent years, transparent conductive films having various kinds of plastic films as bases, including polyethylene terephthalate films, have been favorably used because of the advantages of being excellent in impact resistance in addition to flexibility and processability and being lightweight.
When a transparent conductive layer such as an ITO film is formed on a plastic film base, sputter deposition cannot be performed at a high temperature because of the limitation by the heat resistance of the base. Thus, ITO immediately after film formation forms an amorphous film (which may be partially crystallized). Such an amorphous ITO film has the problems that it is highly yellowish and poor in transparency, a change in resistance after a humidification heat test is large, and so on.
Thus, generally, an amorphous ITO film is converted into a crystalline ITO film by heating under an oxygen atmosphere in the air after the amorphous ITO film is formed on a film base formed of a polymer molded product (see, for example, Patent Document 1). This method provides the advantages that the transparency of the ITO film is improved, and a change in resistance after the humidification heat test is small, so that humidification heat reliability is improved.
On the other hand, a transparent conductive film using a film base has such a problem that a transparent conductive layer is poor in scratch resistance, and is therefore scratched during use to increase the electric resistance or cause breaking. In particular, in a transparent conductive film for a touch panel, a pair of thin films made to face each other with a spacer interposed therebetween are strongly abutted against each other by press dotting from the panel plate side of one of the thin films, and therefore satisfactory endurance property, i.e. dotting property, capable of being resistant thereto is desired. However, a transparent conductive film using a film base is generally inferior in dotting property to an electroconductive glass, and therefore has the problem that the lifetime is decreased as a touch panel.
In view of the aforementioned problem, a transparent conductive film formed by providing a film base having a specific thickness, forming on one surface thereof a transparent dielectric thin film having an optical refractive index smaller than the optical refractive index of the film base, and further forming thereon a transparent conductive layer in this order, and bonding another transparent substrate to the other surface of the film base with a pressure-sensitive adhesive layer interposed therebetween has been proposed (Patent Document 2). According to such a transparent conductive film, the transparency and the scratch resistance of the transparent conductive layer can be improved, and the dotting property as intended for use in a touch panel are improved. Dotting property, when a touch panel is used in a bent state, are improved by forming a transparent conductive layer on one surface of a transparent film base with a plurality of dielectric thin films interposed therebetween (Patent Document 3).
On the other hand, a touch panel of a game machine is often hard hit as compared to other applications, and therefore a transparent conductive film for use therein is required to retain dotting property under a heavier load. Further, dotting property and sliding durability at the screen edge are required as the frame of the touch panel is narrowed, but in input operations at the screen edge, the transparent conductive film is more sharply bent as compared to input operations at the central part of the screen. Therefore, the transparent conductive film is required to retain a higher bending resistance in addition to dotting property under a heavy load.